Zombie Panic! Source is a standalone cooperative, survival-horror, first-person-shooter! Set in the middle of a zombie apocalypse, players start as a small group of survivors attempting to stay alive. Each map will have its own objectives to complete and win the round or survive by a period of time.
ZPS v2.0 Release Date and Halloween Contest
That's right out of Left 4 Dead 2 think of something more original! I've played this mod and its team choice system is counter-intuitive I cannot figure out how to play as survivor. The gameplay isn't too bad but the mapping is awful. I long for better quality maps for this game then I might play it.
This was out LONG before Left 4 Dead was. You press F1 for survivor and F2 for zombie. You can also press F3 for spectator mode.
Official ZPS maps are complex and are well done. You have to work as a TEAM. You go it alone you will die. You have a stamina bar and can only hold so much ammo and guns before you get bogged down with the weight.